Footwear, particularly boots and shoes intended for industrial application, commonly are made by molding a sole of plastics material directly around the lower edge of the upper, the molding operation also joining the two parts together. In this specification and the appended claims the term "sole" as applied to footwear embraces the entire lower portion of such footwear namely the forward sole portion and the rearward heel portion. Foamed polyurethane plastics materials commonly are used for this purpose and generally are found to be very satisfactory, although there are some applications in which it would be preferable to use alternative materials, such as rubber. For example, in iron and steel industry plants there is increased possibility that the footwear will be subjected to temperatures high enough to soften a polyurethane sole, or even high enough to melt the material, e.g. if the wearer steps on a hot piece of metal. Another example arises when the footwear is used in very cold temperatures, the polyurethane becoming hard, brittle and somewhat slippery underfoot. Rubber materials can have higher abrasion resistance than polyurethanes and are more satisfactory under these conditions, but generally are also much more expensive, to the extent that footwear with a sole entirely of rubber would be more expensive than is generally acceptable for this type of product. One solution to the problem has been to make the footwear with a sole in which the major part is of polyurethane, while a thin outer layer forming the sole tread is of rubber.
Apparatus for the production of footwear with molded soles almost universally comprises a turntable with a plurality of molds spaced uniformly around the turntable circumference at a corresponding number of stations. At a loading station a boot upper is mounted on a last which is engaged with the remainder of the respective mold to close its sole forming cavity; the plastics material is then injected to fill the cavity. The turntable is indexed while the plastics material polymerizes and hardens and while successive stations are loaded until the molding operation is complete and the finished boot or shoe can be removed at an unloading station; the cycle then repeats. In one known process for the production of footwear comprising such a laminated sole a thin flat layer of a first material, usually a vulcanisable rubber compound, is loaded into the mold and engaged by a dummy last of the shape required to cooperate with the mold in forming the tread layer alone; the turntable is then rotated through a first cycle during which the layer is vulcanised under heat and pressure and molded to the required shape. At the end of the first cycle the dummy last is replaced by a last carrying a boot or shoe upper and the turntable indexed through a second cycle during which the remainder of the sole is formed and the boot or shoe completed. Such a two cycle process lowers the output available from each machine, as compared to one producing footwear in which the sole is entirely of a single material, and correspondingly increases the cost of production. In another prior art process the sole tread parts are first molded to shape from a cured rubber compound, using molds with the required thin mold cavities and with the tread patterns required in the finished footwear, so that the molded tread parts can be fitted by the operator into the footwear producing molds prior to closing them and injection of the plastics material therein. Such a process requires the use either of a separate machine, or of a footwear producing machine, during the period taken by the curing and molding, and in addition requires the provision of a separate set of treadforming molds for each pair of boots or shoes, for each size to be produced, and for each tread pattern required.